In the basic settlers game, there’s not a whole lot of advanced strategy, although there is some. Once you start adding in the expansions, it gets a lot more interesting. I don’t even bother playing settlers anymore without at least the seafarers expansion.
But in basic settlers, a few things to watch for are:
Look for places where there’s a high probability number near a port of the same resource. 2:1 trades are real nice especially when you rake in that resource on a 6 or 8.
Go for number diversity as well as resource diversity. Despite all probability, it is still a dice game, and random strings of luck happen. Long stretches without a 6, or 10 5’s in a row can and do happen. So I like to make sure I minimize the chances of getting nothing, by claiming as many numbers as possible. (e.g., 5 and 9 are theoretically equal, but if you have a 5 already, take the 9.)
Trading is the big key. Don’t trade unless it helps you, and preferably helps you immediately, so you can build before the robber steals away what you just traded for. Don’t trade with anyone who has 9 points. Act pathetic when you’re behind, and people may give you more stuff. Drive hard bargains, especially for things that are in shortage, trying to get at least 2 cards for 1 prized resource if it’s been shut down for a while.
Don’t forget the power of the cards. Monopoly is DEVASTATING if timed right. Especially if you do stuff like Innocently asking, “Anyone have wheat to trade? Do you have wheat? Do you have wheat?” Then if you get a yes, or a “not that I can trade” call monopoly on wheat and cackle maniacly.
Realize the relative value of the different resources. In basic settlers, sheep are the least useful. Brick/Clay and Wood are useful early game, with Wheat and Rock more useful in the late game. So if your initial start has to be missing something, it’s safer to miss rock or wheat, than wood or brick.